X-rays are commonly used in medical and dental imaging techniques for examining living things, as well as in internal examination of objects in materials analysis and other fields. X-rays are commonly passed through the object to be imaged, such as a person or a metal casting, and the X-rays that are not absorbed and pass through the object are recorded on a medium, such as a photographic film or a semiconductor detector.
X-rays generally travel in straight lines directly between an X-ray source, through the object to be imaged, and to the detector. However, the clarity and resolution of the image may be degraded by X-rays that have a distorted or bent path, due to being scattered or deflected away from the usual straight path rather than simply being absorbed, for example, being scattered by a bone. In this case any particular portion of the X-ray detector will record some X-rays that have not travelled to the detector in a straight line, which will represent a source of ‘noise’, degrading the signal to noise ratio (S/N) of the image. The ‘noise’ may reduce the sharpness of the image and result in an image that does not provide a clear view of the features to be imaged.
A method of reducing the number of X-rays that do not travel directly from the X-ray source to the detector includes the use of thin sheets of an X-ray opaque material such as lead, separated by sheets of an X-ray transparent (also referred to as X-ray lucent) material such as aluminum, to form a structure similar to a Venetian Blind. This structure reduces the number of X-rays that travel to the detector with greater than a specific blocking angle to the vertical lead sheets, where the blocking angle is determined by a ratio between the height (or depth D) of the vertical lead sheets and the separation (L) between the vertical sheets (i.e., an L/D ratio). The thickness of the lead sheets must also be great enough to block X-rays of the energy level being used.
It is to be understood that since the lead/aluminum sheet method uses lead sheets to form a linear array, the blocking angle is only applicable in the direction perpendicular to the linear array, and that it would require a second such linear array placed on top of the first, and rotated ninety degrees relative to the first linear array, to form a grid pattern to obtain a general X-ray anti-scatter device. In general, the grid is placed somewhere between the object to be examined and the detector.